hey!
i am trying to sew a vest, and too late i have realized that i have no clue how should i make the curved hems for the shoulder and neck areas.
i've tried looking for some information on google, but haven't found much of anything i could understand.

it's not even the sewing that i am stuck with, it's the actual patterning of the hem. should i just make the curved hem as an extra piece of fabric like i'd make a normal hem? but then it would feel weird and i'll never manage to get the right curved shape after i'll try to fold it.

If you're not fully lining your vest, you could do what's called a facing -- cut a portion of your pattern at the armhole (it looks like that's what you've drawn), truncating it several inches in, and sew it together to your face fabric at the armhole before turning that to the inside. Sometimes facings are left loose, and sometimes they're blind-tacked down to the face fabric. You'll find similar things in the necklines of some shirts and dresses. I think that's what you're trying to do from your description ... You can sometimes get a miniscule rolled hem on a curve like that, but facings are more cooperative in most cases.

(Sorry, I'm not particularly coherent today; hope that helps a little, or at least gives you a better jumping-off point.)

If you aren't lining it, use bias tape. Bias tape CAN go around curves and you don't need to build it into your pattern.

Though for a vest, you really should be lining and facing it otherwise it'll have little stability. Vests need a certain amount of stiffness and reinforcement at the edges. With stuff like this, I do a full bag-lining and put strips of interfacing around all the edges. (Bag-lining = basically make two versions of the garment, sew them right sides together most of the way around, then turn it right side out. Iron it flat then top-stitch in place.) If you want the edging to stand out, you could interface/line it and just bind it all together at the edges with bias tape instead of sewing it inside out.

Facings aren't that complex. Less-so than a lining when it comes to vests, because vest linings have to be done differently than regular bag-out lining (excuse me while I pause to curse the side seams in vests!). Bias tape may be simpler (or at least faster) but if your vest's design DOESN'T have a band of fabric or a ridge around the edge of the collar and armholes, or around the whole thing really, then putting one on might be inaccurate and it might look funny. If it's an original design, that's probably not a problem, but if you're reproducing a costume from a series or movie, you have to take that into account.

__________________Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild

I'm in the facings camp, they are actually EASIER than trying to manipulate bias around a tight curve like an arm hole. Also, a facing can be made lots wider than bias tape comes, which means it'll lay flatter when completed. bias tape on a tight curve will likely make it pucker.

it seems like just a piece of fabric that i attach behind my visible fabric, so how does it actually help me get the edges look good?

Because you sew the pieces right sides together and then turn it right side out, totally enclosing the raw edge of the fabric.

To get a smooth curve, you need to snip little notches in the seam allowance almost up to the stitching line. Then after you flip it right side out, iron it flat and then topstitch the edge to keep it all in place.